The present invention relates to a polyamic acid, a polyimide and a process for preparing them, for which is used a diamine isomer mixture, as well as to a varnish containing the polyamic acid and a film containing the polyimide. Specifically, the invention relates to a polyimide having excellent thermal resistance, melt flowability, optical properties and chemical resistance and further controlled dielectric properties in addition to excellent physical properties inherent to polyimides, that is, thermal resistance, mechanical properties, slidability, low water absorption, electric properties and radiation resistance, and relates to a polyamic acid being a precursor of the polyimide, to a process for preparing them, and to a varnish or film thereof.
Conventionally, polyimides have been widely used as the molding materials, composite materials and electric materials in various fields for excellent mechanical properties and electronic properties in addition to it""s excellent thermal resistance.
For example, the polyimide represented by the formula (A): 
has been known as a typical polyimide. However, the polyimide is non-thermoplastic, therefore the polyimide has a significant drawback in moldability because of insolubility and infusibility, and there is problem that mass production of its moldings is substantially impossible. One concrete method of processing the polyimide comprises forming it into a mass in a special molding process of powder sinter molding followed by mechanically processing it, by cutting, machining or polishing it to give a shaped article. Films of the polyimide to be used in the field of electronic materials have good thermal resistance and mechanical properties, but their properties are not satisfactory in the field where high-frequency waves are used. In addition, the films are yellowish brown, and therefore cannot be used for optical materials.
The polyimide represented by the formula (B) 
is also non-thermoplastic, and its use is generally for films, which are mainly used in the field of electronic materials. The polyimide is excellent in thermal resistance and mechanical properties, but its films are yellowish brown and therefore cannot be used for optical materials.
The polyetherimide represented by the formula (C): 
and the polyimide represented by the formula (D): 
have been known as a polyimide having an improved moldability and workability.
These polyimides have good properties such as thermal resistance inherent to polyimides, and are generally used for molding materials. However, these are also pale yellow to brown, and are therefore not used for optical materials.
On the other hand, various materials have been developed with the development in optical communication, and materials substitutable for quartz are much studied. As well known, their typical examples are polymethyl methacrylates and polycarbonates. In addition, cyclic polyolefins represented by the formulae (E) and (F) and fluorine-containing polymers represented by the formula (G) have been developed. 
These polymers have excellent optical properties, and their applications are extending for optical fibers, optical wave-guides, optical disc substrates, optical lenses, optical filters, etc. However, the polymers all have a glass transition temperature not higher than about 180xc2x0 C., and their thermal resistance is unsatisfactory for their use at high temperatures.
Apart from the optical materials mentioned above, electric insulating materials usable in a high frequency region are desired with the development of electronic instruments. According to this, low-dielectric and low-loss materials are being developed. In molecular planning for these, it is said that introducing a fluoro group or a trifluoromethyl group into the main chain skeleton of polymers is indispensable. For example, A. K. St. Clair, et al. of the US National Aeronautics and Space Administration (NASA) has been disclosed various polyimides in Polymeric Materials Science and Engineering, Vol. 59, p. 28 (1988) and EP 0299865; and Yamashita, et al. in U.S. Pat. Nos. 5,354,839 and 5,410,084. For these polyimides, however, extremely expensive starting materials must be used for introducing the essential substituent thereinto, and this is a great obstacle to the practical application of the polyimides.
On the other hand, some methods have been tried for improving the optical properties such as colorless transparency and the electric properties of polyimides without introducing such expensive fluorine thereinto. For example, alicyclic diamine compounds or acid dianhydrides are used for developing polyimides having improved optical properties such as colorless transparency. In the prior art of JP-A 7906/1998 or WO98/29471, used is a mixture of alicyclic diamine compounds, 2,5-diaminomethyl-bicyclo[2.2.1]heptane (hereinafter abbreviated as 2,5-NBDA) and 2,6-diaminomethyl-bicyclo[2.2.1]heptane (hereinafter abbreviated as 2,6-NBDA) to obtain polyimides of good light transmittance (high colorless transparency).
As so mentioned in that WO98/29471, however, these NBDAs used in the prior art are 2,5-substituted and 2,6-substituted isomers and are extremely difficult to separate, and therefore a mixture of the two is directly used as it is. The polyimides formed from these NBDAs are surely better than conventional polyimides in point of their optical properties, but further polyimides having better film properties and better optical properties have been desired in the art.
The object of the present invention is to provide a polyimide having further improved and controlled properties in thermal resistance, melt flowability, optical properties, chemical resistance and electric properties in addition to excellent physical properties inherent to polyimides, that is, thermal resistance, mechanical properties, slidability, low water absorption, electric properties, radiation resistance, and to provide its precursor, a polyamic acid. In addition, the object of the invention is to provide a process for preparing them, and to provide a varnish and a film comprising them, which are important embodiments of their practical use.
The present inventors have assiduously studied so as to attain the above objects.
As a result, the inventors have already found a process for preparing an alicyclic diamine, diaminomethyl-bicyclo[2.2.1]heptane (hereinafter abbreviated as NBDA), in which the composition ratio of the 2,5-substituted isomer and the 2,6-substituted isomer in the diamine isomer mixture can be varied. In addition, the inventors have clarified that the 2,5-substituted isomer and the 2,6-substituted isomer additionally form stereo-isomers, that is, they are in the form of a mixed composition of four structural isomers, (2S,5S)-NBDA, (2S,5R)-NBDA, (2S,6R)-NBDA and (2S,6S)-NBDA. With that, the inventors have further studied the compositions of these NBDA isomers and the properties of the polyamic acids and the polyimides produced from them, and have found out the polyimide of the present invention which naturally has the good properties inherent to polyimides and additionally has further improved and controlled thermal resistance, melt flowability, optical properties, chemical resistance, electric properties and film properties, depending on the composition ratio of the starting isomers used.
Specifically, the present invention relates to the following:
1) A polyamic acid having repeating units represented by the formula (I): 
wherein the norbornane skeleton of 
comprises four components of 
and their contents satisfy the following:
1%xe2x89xa62,5-[diexo]xe2x89xa690%,
1%xe2x89xa62,5-[exo,endo]xe2x89xa690%,
1%xe2x89xa62,6-[diexo]xe2x89xa690%,
1%xe2x89xa62,6-[exo,endo]xe2x89xa690%,
provided that
(2,5-[diexo])+(2,5-[exo,endo])+(2,6-[diexo])+-[exo,endo])=100%,
R represents a tetravalent group having from 4 to 27 carbon atoms and selected from the group consisting of an aliphatic group, a monocyclic aliphatic group, a condensed polycyclic aliphatic group, a monocyclic aromatic group, a condensed polycyclic aromatic group, and a non-condensed polycyclic aliphatic or aromatic group which is composed of cycloaliphatic or aromatic groups mutually bonded to each other either directly or via a crosslinking member;
2) A polyamic acid having repeating units represented by the formula (1): 
wherein the norbornane skeleton of 
comprises four components of 
and their contents satisfy the following:
10%xe2x89xa62,5-[diexo]xe2x89xa640%,
10%xe2x89xa62,5-[exo,endo]xe2x89xa640%,
10%xe2x89xa62,6-[diexo]xe2x89xa640%,
10%xe2x89xa62,6-[exo,endo]xe2x89xa640%,
provided that
(2,5-[diexo])+(2,5-[exo,endo])+(2,6-[diexo])+(2,6-[exo,endo])=100%,
R represents a tetravalent group having from 4 to 27 carbon atoms and selected from the group consisting of an aliphatic group, a monocyclic aliphatic group, a condensed polycyclic aliphatic group, a monocyclic aromatic group, a condensed polycyclic aromatic group, and a non-condensed polycyclic aliphatic or aromatic group which is composed of cycloaliphatic or aromatic groups mutually bonded to each other either directly or via a crosslinking member;
3) A polyamic acid having repeating units represented by the formula (1): 
wherein the norbornane skeleton of 
comprises four components of 
and their contents satisfy the following:
20%xe2x89xa62,5-[diexo]xe2x89xa630%,
20%xe2x89xa62,5-[exo,endo]xe2x89xa630%,
20%xe2x89xa62,6-[diexo]xe2x89xa630%,
20%xe2x89xa62,6-[exo,endo]xe2x89xa630%,
provided that
(2,5-[diexo])+(2,5-[exo,endo])+(2,6-[diexo])+(2,6-[exo,endo])=100%,
R represents a tetravalent group having from 4 to 27 carbon atoms and selected from the group consisting of an aliphatic group, monocyclic aliphatic group, a condensed polycyclic aliphatic group, a monocyclic aromatic group, a condensed polycyclic aromatic group, and a non-condensed polycyclic aliphatic or aromatic group which is composed of cycloaliphatic or aromatic groups mutually bonded to each other either directly or via a crosslinking member;
4) A polyimide having repeating units represented by the formula (2): 
wherein the norbornane skeleton of 
comprises four components of 
and their contents satisfy the following:
1%xe2x89xa62,5-[diexo]xe2x89xa690%,
1%xe2x89xa62,5-[exo,endo]xe2x89xa690%,
1%xe2x89xa62,6-[diexo]xe2x89xa690%,
1%xe2x89xa62,6-[exo,endo]xe2x89xa690%,
provided that
(2,5-[diexo])+(2,5-[exo,endo])+(2,6-[diexo])+(2,6-[exo,endo])=100%,
R represents a tetravalent group having from 4 to 27 carbon atoms and selected from the group consisting of an aliphatic group, a monocyclic aliphatic group, a condensed polycyclic aliphatic group, a monocyclic aromatic group, a condensed polycyclic aromatic group, and a non-condensed polycyclic aliphatic or aromatic group which is composed of cycloaliphatic or aromatic groups mutually bonded to each other either directly or via a crosslinking member;
5) A polyimide having repeating units represented by the formula (2): 
wherein the norbornane skeleton of 
comprises four components of 
and their contents satisfy the following:
10%xe2x89xa62,5-[diexo]xe2x89xa640%,
10%xe2x89xa62,5-[exo,endo]xe2x89xa640%,
10%xe2x89xa62,6-[diexo]xe2x89xa640%,
10%xe2x89xa62,6-[exo,endo]xe2x89xa640%,
provided that
(2,5-[diexo])+(2,5-[exo,endo])+(2,6-[diexo])+(2,6-[exo,endo])=100%,
R represents a tetravalent group having from 4 to 27 carbon atoms and selected from the group consisting of an aliphatic group, a monocyclic aliphatic group, a condensed polycyclic aliphatic group, a monocyclic aromatic group, a condensed polycyclic aromatic group, and a non-condensed polycyclic aliphatic or aromatic group which is composed of cycloaliphatic or aromatic groups mutually bonded to each other either directly or via a crosslinking member;
6) A polyimide having repeating units represented by the formula (2): 
wherein the norbornane skeleton of 
comprises four components of 
and their contents satisfy the following:
20%xe2x89xa62,5-[diexo]xe2x89xa630%,
20%xe2x89xa62,5-[exo,endo]xe2x89xa630%,
20%xe2x89xa62,6-[diexo]xe2x89xa630%,
20%xe2x89xa62,6-[exo,endo]xe2x89xa630%,
provided that
(2,5-[diexo])+(2,5-[exo,endo])+(2,6-[diexo])+(2,6-[exo,endo])=100%,
R represents a tetravalent group having from 4 to 27 carbon atoms and selected from the group consisting of an aliphatic group, a monocyclic aliphatic group, a condensed polycyclic aliphatic group, a monocyclic aromatic group, a condensed polycyclic aromatic group, and a non-condensed polycyclic aliphatic or aromatic group which is composed of cycloaliphatic or aromatic groups mutually bonded to each other either directly or via a crosslinking member;
7) A process for preparing a polyamic acid, which comprises reacting a mixture of diaminomethyl-bicyclo[2.2.1]heptanes, (2S,5S)-diaminomethyl-bicyclo[2.2.1]heptane represented by the formula (3-1): 
(2S,5R)-diaminomethyl-bicyclo[2.2.1]heptane represented by the formula (3-2): 
(2S,6R)-diaminomethyl-bicyclo[2.2.1]heptane represented by the formula (3-3): 
and (2S,6S)-diaminomethyl-bicyclo[2.2.1]heptane represented by the formula (3-4): 
wherein,
1%xe2x89xa6(2S,5S)-diaminomethyl-bicyclo[2.2.1]heptanexe2x89xa690%,
1%xe2x89xa6(2S,5R)-diaminomethyl-bicyclo[2.2.1]heptanexe2x89xa690%,
1%xe2x89xa6(2S,6R)-diaminomethyl-bicyclo[2.2.1]heptanexe2x89xa690%,
1% less than (2S,6S)-diaminomethyl-bicyclo[2.2.1]heptanexe2x89xa690%,
provided that,
(2S,5S)isomer+(2S,5R)isomer+(2S,6R)isomer+(2S,6S)isomer=100%,
with a tetracarboxylic dianhydride represented by the formula (4); 
wherein R represents a tetravalent group having from 4 to 27 carbon atoms and selected from the group consisting of an aliphatic group, a monocyclic aliphatic group, a condensed polycyclic aliphatic group, a monocyclic aromatic group, a condensed polycyclic aromatic group, and a non-condensed polycyclic aliphatic or aromatic group which is composed of cycloaliphatic or aromatic groups mutually bonded to each other either directly or via a crosslinking member;
8) A process for preparing a polyamic acid, which comprises reacting a mixture of diaminomethyl-bicyclo[2.2.1]heptanes, (2S,5S)-diaminomethyl-bicyclo[2.2.1]heptane represented by the formula(3-1): 
(2S,5R)-diaminomethyl-bicyclo[2.2.1]heptane represented by the formula (3-2): 
(2S,6R)-diaminomethyl-bicyclo[2.2.1]heptane represented by the formula (3-3): 
and (2S,6S)-diaminomethyl-bicyclo[2.2.1]heptane represented by the formula (3-4): 
wherein
10%xe2x89xa6(2S,5S)-diaminomethyl-bicyclo[2.2.1]heptanexe2x89xa640%,
10%xe2x89xa6(2S,5R)-diaminomethyl-bicyclo[2.2.1]heptanexe2x89xa640%,
10%xe2x89xa6(2S,6R)-diaminomethyl-bicyclo[2.2.1]heptanexe2x89xa640%,
10%xe2x89xa6(2S,6S)-diaminomethyl-bicyclo[2.2.1]heptanexe2x89xa640%,
provided that,
(2S,5S)isomer+(2S,5R)isomer+(2S,6R)isomer+(2S,6S)isomer=100%,
with a tetracarboxylic dianhydride represented by the formula (4): 
wherein R represents a tetravalent group having from 4 to 27 carbon atoms and selected from the group consisting of an aliphatic group, a monocyclic aliphatic group, a condensed polycyclic aliphatic group, a monocyclic aromatic group, a condensed polycyclic aromatic group, and a non-condensed polycyclic aliphatic or aromatic group which is composed of cycloaliphatic or aromatic groups mutually bonded to each other either directly or via a crosslinking member;
9) A process for preparing a polyamic acid, which comprises reacting a mixture of
diaminomethyl-bicyclo[2.2.1]heptanes,
(2S,5S)-diaminomethyl-bicyclo[2.2.1]heptane represented by the formula (3-1): 
(2S,5R)-diaminomethyl-bicyclo[2.2.1]heptane represented by the formula (3-2): 
(2S,6R)-diaminomethyl-bicyclo[2.2.1]heptane represented by the formula (3-3): 
and (2S,6S)-diaminomethyl-bicyclo[2.2.1]heptane represented by the formula (3-4): 
wherein
20%xe2x89xa6(2S,5S)-diaminomethyl-bicyclo[2.2.1]heptanexe2x89xa630%,
20%xe2x89xa6(2S,5R)-diaminomethyl-bicyclo[2.2.1]heptanexe2x89xa630%,
20%xe2x89xa6(2S,6R)-diaminomethyl-bicyclo[2.2.1]heptanexe2x89xa630%,
20%xe2x89xa6(2S,6S)-diaminomethyl-bicyclo[2.2.1]heptanexe2x89xa630%,
provided that,
(2S,5S)isomer+(2S,5R)isomer+(2S,6R)isomer+(2S,6S)isomer=100%,
with a tetracarboxylic dianhydride represented by the formula (4): 
wherein R represents a tetravalent group having from 4 to 27 carbon atoms and selected from the group consisting of an aliphatic group, a monocyclic aliphatic group, a condensed polycyclic aliphatic group, a monocyclic aromatic group, a condensed polycyclic aromatic group, and a non-condensed polycyclic aliphatic or aromatic group which is composed of cycloaliphatic or aromatic groups mutually bonded to each other either directly or via a crosslinking member;
10) A process for preparing a polyimide, which comprises thermally or chemically imidizing the polyamic acid obtained in the above 7;
11) A process for preparing a polyimide, which comprises thermally or chemically imidizing the polyamic acid obtained in the above 8;
12) A process for preparing a polyimide, which comprises thermally or chemically imidizing the polyamic acid obtained in the above 9;
13) The polyamic acid of above 1, 2 or 3, of which the inherent viscosity measured in a solvent of N-methyl-2-pyrrolidone having the acid concentration of 0.5 g/dl at 35xc2x0 C. is in the range of 0.1 to 3.0 dl/g;
14) The polyimide of above 4, 5 or 6, of which the inherent viscosity measured in a mixed solvent of p-chlorophenol/phenol=9/1 (by weight) having the polyimide concentration of 0.5 g/dl at 35xc2x0 C. is in the range of 0.1 to 3.0 dl/g;
15) A polyamic acid varnish containing the polyamic acid of above 1;
16) A polyamic acid varnish containing the polyamic acid of above 2;
17) A polyamic acid varnish containing the polyamic acid of above 3;
18) A polyimide film containing the polyimide of above 4;
19) An amorphous polyimide film containing the polyimide of above 5;
20) An amorphous polyimide film of improved smoothness, containing the polyimide of above 6.